A secondary electron detector unit is known from Polish patent application no. 329339, entitled “High-Pressure Scanning Electron Microscope” and from U.S. patent application Ser. No. 11/248,431, filed Oct. 13, 2005 (Pub. No. US 2006/0027748 A1), and entitled “Secondary Electron Detector Unit for a Scanning Electron Microscope”. This secondary electron detector unit comprises a microporous plate (advantageously, of the microsphere type) and a secondary electron detector of the scintillator type. In both designs mentioned, a secondary electron stream is introduced to the aperture in the lower vacuum wall which is biased at a proper voltage.
The aperture constitutes the lower throttling aperture that limits gas flow from the specimen chamber to the intermediate chamber. Secondary electrons that come into the intermediate chamber are directed toward the input surface of the microporous plate placed at the beam axis and are multiplied on their way through the plate. On the output side of the microporous plate, this amplified stream of the secondary electrons is attracted by the scintillator biased at a high voltage and is finally detected, that is, it is converted into a light signal passing through the light guide to the photomultiplier to be converted into an electric signal once more.
Detectors of the type described cannot detect backscattered electrons efficiently because they have too high energies to be focused in the lower throttling aperture and get into the intermediate chamber. Thus, the output signal of the detector does not contain full information about the specimen, because backscattered electrons carry information as to its material composition while secondary electrons provide information as to its topography.